Throughout the industrial arts, there are numerous instances where heat must be applied to work and such heat is most effectively and efficiently applied to the work by flexible electric resistance blanket heaters. For example, in the course of fabricating fluid conducting systems of polyvinylchloride tube stock or pipe, it is often required that sections of the pipe be bent. The bending of such pipe is commonly achieved by engaging a flexible blanket heater about the portion of the pipe to be bent and the plastic material of the pipe is heated to about 300.degree. F., at which temperature it becomes sufficiently plastic to be manually bent, as desired. In such a case, after the pipe is suitably heated, bent, the heater is removed from the pipe and the pipe is suitably bent.
Blanket heaters for the above particular use and for most other special uses must often be extremely flexible to enable them to be effectively engaged with their related work, must be capable of being heated up and cooled rapidly in order to conserve of both electric current and time; and must be capable of being repeatedly wrapped, unwrapped, bent, twisted and otherwise worked throughout as long a service life as can be afforded.
In the art of industrial blanket heaters of the general character referred to above and here concerned with, the most effective, efficient and dependable resistance elements are multi-strand wire elements twisted in a gentle helix to allow for limited elongation to relieve tension and being repeatedly flexed and closely bent, back and forth without stretching and and to minimize hardening of the strands or filaments of which they are made.
The most common prior art industrial blanket heater structure of the type or class here concerned with are established by arranging the twisted wire elements in predetermined patterns (serpentine, zig-zag or the like) between a pair of sheet-like laminates of flexible, elastic, dielectric material and bonding, fusing or welding the laminates together with the wire element held captive therebetween. To impart such heater structures with desired flexibility and to reduce their mass so as to increase the rate at which they heat up and cool off, laminates are used to establish the thinnest and most flexible blanket structures possible, with practical limits. The thickness, mass and flexibility of such prior art blanket heaters is, however, greatly and/or critically limited by the flexibility of the wire resistance elements thereof. That is, the laminated blankets within which the resistance elements are engaged must be sufficiently thick and/or stiff to prevent the resistance elements within them from being so tightly bent that the strands of the elements are drawn beyond their elastic limits and caused to kink within the heater structures as they are flexed and worked. When a strand or two of resistance elements of the character here concerned with are drawn and kinked within their related blanket structures, the kinks establish fulcrums or bending points in the elements at which all subsequent bending and working of the adjacent portions of the blanket structures tend to concentrate and which results in rapid work-hardening and breaking of the elements and premature self-destruction of the heater structures.
As a result of the foregoing, the practical thickness and resulting mass of the blanket structure of industrial blanket heaters is often a compromise between optimum flexibility, optimum minimum mass and that structural stiffness that must be afforded for the resistance elements in order to prevent drawing or stretching of the elements and resulting kinking and premature working thereof. As a result of the above noted compromise, in a large percentage of industrial heaters that are made sufficiently thin and flexible to minimize their mass and increase their heating and cooling rate and/or to enhance their capacity to conform with related work, the resistance heating elements are highly subject to kinking, work-hardening and breaking, the heaters are inherently short-lived and undependable. In such heaters that are made sufficiently thick and/or stiff to afford adequate support for and prevent stretching, kinking and premature work-hardening of the resistance elements, though more dependable and generally longer lived, are slow to heat and cool and are therefore notably less efficient and economical to use and are frequently too stiff and bulky to effectively conform with related work.
As a result of the foregoing, the great majority of those industrial blanket heaters which are subject to being repeatedly and/or continuously manually manipulated and worked are, at best, comprised structure which are less flexible and slower to heat and cool than would normally be preferred.